1. Field of the Invention
This invention is directed to a respirator intended for connection to human or animal airways.
2. Description of the Prior Art
Conventional respirator treatment makes use of frequencies and tidal volumes which correspond to those encountered in a patient who is breathing spontaneously. In this connection, however, it is difficult in some patients to obtain a satisfactory gas exchange in all parts of the lungs without having to apply injurious pressure which can cause a rupture or affect the blood vessels in such a way as to inhibit blood circulation through the lungs.
To avoid such injurious pressures and provide more uniform ventilation of the lungs, certain proposals for respirator design have been made previously.
One previously known respirator (AGA Bronchovent, AGA Medical AB, Lidingo, l Sweden) is equipped with a tracheal main tube which is provided with a branch tube used to supply the respiratory gas at a pressure of up to 1 kg/cm.sup.2 to the tracheal main tube passage through which the respiratory gas flows, whereupon the branch tube opens into the tracheal main tube via a restrictor which provides high flow resistance. This respirator is said to be more effective than a conventional respirator, probably due to the fact that the respiratory gas is supplied in such a way that turbulence builds up, thereby enhancing gas diffusion. It is also said that it provides better mucus transport and reduces the tendency toward spontaneous breathing. This known respirator, however, entails risk for the patient because of the high pressure that is applied to the respiratory gas. Moreover, it is difficult to humidify the respiratory gas. For this previously known respirator, the breathing frequency has been increased to approximately two breaths per second as compared with a normal breathing rate of 1/2-1/4 breaths per second, and this results in smaller tidal volumes and lower pressure in the lungs., The improved ventilation is probably attributable to the turbulence that occurs in the respiratory gas. For a more detailed description of this known respirator, refer to Anesthesia and Analgesia, Vol. 59, No. 8, August 1980, p. 594-603.
Another previously proposed respirator (Klain Jet Ventilator, Acutronic AG, Jona, Switzerland) uses is called jet injection. Here, gas exchange in the lungs is accomplished by means of a gas jet that is supplied to the lungs in pulses having a frequency of up to 40 Hz through a passage in the tracheal tube or through a slender cannular tube placed directly in the trachea, while expiratory gas leaves the lungs via another passage in the tracheal tube or through the trachea. This respirator provides uniform and low pressure in the lungs, and this is advantageous in connection with surgery, because there is no movement of the thorax and there is minimal effect on blood circulation through the lungs. A respirator of this design, however, has many disadvantages. The gas jet can injure and, in the worst case, penetrate the mucous membranes in the trachea, and it can injure the cilia. It is difficult to humidify the respiratory gas, and it is impossible to ascertain whether or not ventilation is taking place by observing the movement of the chest. Here, there is no easy way to determine how much of the respiratory gas has been in the lungs or to measure the composition of the gas in the alveoli. For information about this respirator, refer to Crit. Care Med. 5:280-287, 1977.
Finally, a respirator has been proposed for which the tracheal tube comprises intersecting tubes through which the respiratory gas passes continuously. A pulsator which operates at right angles to the flowing gas transports the gas through the tracheal duct passage at a frequency of up to 40 or 50 Hz. As far as it is known, this respirator has thus far only been used on an experimental basis for animals and not on humans. Increased transport of mucus was herewith observed, but it has been impossible to check the effect of ventilation, and the effect of the high frequencies on blood circulation and biological substances is unknown. For further information, refer to Proc. Am. Exp. Biol. 38 (part II): 951, 1979; and to Science, Vol. 209, 609 and 610, 1980.
In addition to the aforementioned disadvantages, long-term high frequency ventilation, which occurs in connection with (for example) jet injection and the pulsator method, is disadvantageous due to the fact that the functioning of the lungs deteriorates. For example, the lungs become stiffer and the oxygenation of arterial blood deteriorates, and as a result fluids can accumulate in the pleural sac.